1. Technical Field
The disclosed embodiments relate to ESD protection circuits.
2. Background Information
Integrated circuits can be damaged by high voltage spikes produced by electrostatic discharge (ESD). High static charges can develop on the human body. Consider a situation in which a packaged integrated circuit is free and is not coupled to a printed circuit. Power and ground conductors within the integrated circuit may be resting at a first potential. If a person were charged with a static charge, and then were to touch a terminal of the integrated circuit, the high static voltage charge on body of the person might be discharged quickly through the terminal and into the integrated circuit until the integrated circuit and the human body equalize at a common potential. Such an electrostatic discharge event would momentarily introduce high voltages and high currents into the integrated circuit that may damage the integrated circuit. In one example, the gate dielectric material of a small logic transistor in the integrated circuit is thin and breaks down when a high voltage is momentarily present between its gate electrode and the underlying semiconductor material. During the ESD event, the gate dielectric breaks down and is permanently damaged. When the integrated circuit is later incorporated into a usable product, the product may be defective or inoperable due to the damage done to the integrated circuit during handling.
To prevent this situation, circuits called electrostatic discharge (ESD) protection devices are commonly incorporated into integrated circuits. An ESD protection circuit has circuitry that is able to shunt the momentary high currents of an ESD discharge event while dropping a low, non-destructive voltage. One type of ESD protection circuit is commonly referred to as an ESD “clamp.” If the voltage between a voltage supply terminal and a ground terminal of the integrated circuit starts to increase rapidly as in an ESD event, then the ESD protection device becomes conductive and clamps one terminal to the other (or clamps one internal supply voltage bus to another). The clamping is such that the charge of the high voltage ESD event on one of the terminals is discharged through the ESD protection device and to the other terminal. The ESD event is only of a short duration, so after the ESD event the ESD protection device is no longer conductive. There are situations in which circuitry must be operated at a voltage higher than the rating of the semiconductor field effect transistors (FETs) used in the circuit. In such a case, a technique called “cascading” is used. In this technique, FETs of like polarity are placed in series to allow the circuit to operate above the voltage rating of the individual FETs. Such circuits require a bias voltage level between the positive supply terminal and the negative supply terminal.
FIG. 1 (Prior Art) is a circuit diagram of one conventional cascoded ESD protection circuit. Supply voltage conductor 1 is coupled to a first terminal and ground conductor 2 is coupled to a second terminal, and an intermediate supply 9 or cascode bias voltage is coupled to a third terminal. Assume that all nodes of the circuit are initially at the same potential. If the voltage on conductor 1 increases rapidly with respect to ground potential on conductor 2, then the large N-channel FETs 3 and 4 are made conductive to conduct possible ESD current from conductor 1 to conductor 2. The circuit involves two RC circuits. P-channel transistor 5 functions as a resistor and P-channel transistor 6 functions as a capacitor. P-channel transistors 7 and 8 are coupled together in similar fashion. Consider the RC circuit involving transistors 5 and 6. Initially, the capacitance of transistor 6 is discharged and there is no voltage drop across the capacitance of transistor 6. Node 10 is therefore at a digital low with respect to the supply voltages on leads 11 and 12 of inverter 13. When the voltage between conductors 1 and 9 increases rapidly, the capacitance of transistor 6 is charged relatively slowly through the resistance of transistor 5. As a result, a digital low is present on the input of inverter 13. Inverter 13 outputs a digital high, which causes the gate of large N-channel transistor 3 to be coupled to conductor 1. Large N-channel transistor 3 is made conductive. The lower portion of the circuit of FIG. 1 works in an identical fashion to the upper portion of the circuit. Accordingly, both large N-channel transistors 3 and 4 turn on quickly during an ESD condition when the voltage between conductors 1 and 2 is detected to rise rapidly. Transistors 3 and 4 discharge the static ESD charge, and prevent the voltage between conductors 1 and 2 from reaching high levels that would damage other sensitive circuitry within the integrated circuit.
After a short time, the capacitances of transistors 6 and 8 charge to the point that the voltages on nodes 10 and 14 reach the switching voltages of inverters 13 and 15. The inverters 13 and 15 then switch to output digital logic low values that in turn cause transistors 3 and 4 to be nonconductive. Once the large N-channel transistors 3 and 4 are nonconductive, then a voltage supply supplying VDD can be coupled to conductors 1 and 2 in a normal power-up condition. In a normal power-up condition, the voltage between conductors 1 and 2 does not rise quickly as in an ESD event. The voltages between conductors 1 and 9 and between conductors 9 and 2 increase slowly such that the capacitors of transistors 6 and 8 are always adequately charged and such that the voltage on nodes 10 and 14 remain above the switching voltages of inverters 13 and 15. Inverters 13 and 15 therefore always output digital logic low values. The transistors 3 and 4 therefore remain nonconductive. The voltage between conductors 1 and 2 can be raised in this fashion until the voltage between the conductors 1 and 2 is at the supply voltage VDD level. The ESD protection circuit does not conduct current from conductor 1 to conductor 2 during a normal power-up condition.
There are two common models used to test the adequacy of ESD protection circuits: the Human Body Mode (HBM) and the Charge Device Model (CDM). In the CDM model, the ESD pulses are of high current magnitudes but are of shorter duration than the ESD pulses in the HMB model. Under CDM testing, large N-channel transistors used to conduct ESD current in ESD clamps were noticed to fail. Ballasts were therefore provided and the voltage at which the ESD protection failed was successfully increased. It was, however, recognized that providing the ballasts increased the amount of integrated circuit area consumed by the ESD protection circuits. A P-channel transistor of similar construction, although it had a lower carrier mobility and therefore was made larger to conduct the same amount of ESD current as an N-channel transistor, was not seen to fail in the ESD protection circuit application. The amount of integrated circuit area consumed by the P-channel transistor was sometimes less than the amount of integrated circuit area consumed by a smaller N-channel transistor and its associated ballast. Accordingly, ESD protection circuits came to use P-channel transistors for the large ESD current carrying transistors.
FIG. 2 (Prior Art) is a diagram of an ESD protection circuit employing P-channel transistors P1 and P2 for the large ESD current carrying transistors. During a rapid rise of the voltage between conductor 16 and ground conductor 17, the RC circuit 18 initially provides a digital logic low onto the input lead 19 of inverter 20 with respect to the supply voltages on inverter 20. Inverter 20 therefore outputs a digital logic high onto the input lead 23 of inverter 21, which in turn couples the gate of P1 to the low potential on node 22. Transistor P1 is therefore made conductive. The low potential on node 22 is directly coupled to the gate of transistor P2, so transistor P2 is also conductive. Node 23 is coupled to conductor 16 by inverter 20, so node 23 has a higher potential than does node 22. The voltage on node 22 is below the switching threshold of inverter 23. Inverter 24 therefore outputs a digital high voltage (the voltage on node 23) onto the gate of transistor 25, thereby making transistor 25 conductive and keeping the voltage on node 22 coupled to ground potential. The ESD current is conducted from conductor 16, through transistors P1 and P2, and to ground conductor 17.
After the passing of the ESD event, the voltage on node 19 increases with respect to the voltage on conductor 22 to the point that the switching threshold of inverter 20 is reached. Inverter 20 switches, inverter 21 switches, and the gate of transistor P1 is coupled to conductor 16 by inverter 21. Transistor P1 is then turned off. At this time, node 23 is coupled to node 22 by the pulldown transistor in inverter 20. The voltage on the input lead of inverter 24 is no longer below the switching point of inverter 24. Inverter 24 therefore switches and couples the gate of transistor 25 to ground conductor 17, thereby turning transistor 25 off. Because node 22 is no longer coupled to ground conductor 17, the voltage on node 22 rises and turns transistor P2 off. Accordingly, after the ESD event both large P-channel transistors P1 and P2 are nonconductive. Under normal operating conditions with a voltage applied to supply conductor 16, an intermediate voltage to conductor 22, and ground to ground conductor 17, the gate of transistor P1 is held to its source potential thereby biasing transistor P1 off. The gate of transistor P2 is held at the potential of conductor 22, thereby lowering the drain-to-source potential of transistor P1 to a safe level.